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Abiotic contribution to total soil CO2 flux across a broad range of land-cover types in a desert region

Ma, Jie, Liu, Ran, Li, Yan
Journal of Arid Land 2017 v.9 no.1 pp. 13-26
arid lands, carbon, carbon dioxide, carbon sinks, cotton, ecosystems, gardens, halophytes, hops, land cover, soil temperature, China
As an important component of ecosystem carbon (C) budgets, soil carbon dioxide (CO₂) flux is determined by a combination of a series of biotic and abiotic processes. Although there is evidence showing that the abiotic component can be important in total soil CO₂ flux (R ₜₒₜₐₗ), its relative importance has never been systematically assessed. In this study, after comparative measurements of CO₂ fluxes on sterilized and natural soils, the R ₜₒₜₐₗ was partitioned into biotic flux (R bᵢₒₜᵢc) and abiotic flux (R ₐbᵢₒₜᵢc) across a broad range of land-cover types (including eight sampling sites: cotton field, hops field, halophyte garden, alkaline land, reservoir edge, native saline desert, dune crest and interdune lowland) in Gurbantunggut Desert, Xinjiang, China. The relative contribution of Rabiotic to R ₜₒₜₐₗ , as well as the temperature dependency and predominant factors for R ₜₒₜₐₗ , R bᵢₒₜᵢc and R ₐbᵢₒₜᵢc , were analyzed. Results showed that R ₐbᵢₒₜᵢc always contributed to R ₜₒₜₐₗ for all of the eight sampling sites, but the degree or magnitude of contribution varied greatly. Specifically, the ratio of R ₐbᵢₒₜᵢc to R ₜₒₜₐₗ was very low in cotton field and hops field and very high in alkaline land and dune crest. Statistically, the ratio of R ₐbᵢₒₜᵢc to R ₜₒₜₐₗ logarithmically increased with decreasing R bᵢₒₜᵢc , suggesting that R ₐbᵢₒₜᵢc strongly affected R ₜₒₜₐₗ when R bᵢₒₜᵢc was low. This pattern confirms that soil CO₂ flux is predominated by biotic processes in most soils, but abiotic processes can also be dominant when biotic processes are weak. On a diurnal basis, R ₐbᵢₒₜᵢc cannot result in net gain or net loss of CO₂, but its effect on transient CO₂ flux was significant. Temperature dependency of R ₜₒₜₐₗ varied among the eight sampling sites and was determined by the predominant processes (abiotic or biotic) of CO₂ flux. Specifically, R bᵢₒₜᵢc was driven by soil temperature while R ₐbᵢₒₜᵢc was regulated by the change in soil temperature (ΔT). Namely, declining temperature (ΔT<0) resulted in negative R ₐbᵢₒₜᵢc (i.e., CO₂ went into soil) while rising temperature (ΔT>0) resulted in positive R ₐbᵢₒₜᵢc (i.e., CO₂ released from soil). Without recognition of R ₐbᵢₒₜᵢc , R bᵢₒₜᵢc would be overestimated for the daytime and underestimated for the nighttime. Although R ₐbᵢₒₜᵢc may not change the sum or the net value of daily soil CO₂ exchange and may not directly constitute a C sink, it can significantly alter the transient apparent soil CO₂ flux, either in magnitude or in temperature dependency. Thus, recognizing the fact that abiotic component in R ₜₒₜₐₗ exists widely in soils has widespread consequences for the understanding of C cycling.